1. Field of the Invention
The present invention relates to a magnetic sensor and a magnetic recording/reproducing apparatus utilizing a micro magnetic oscillation element.
2. Description of the Related Art
The recording density of magnetic recording media has been improving at a rate of 100% per year since the advent of GMR heads utilizing the giant magneto-resistive effect (GMR effect). A GMR element, such as GMR head, comprises a multi-layer film having a sandwich structure comprising a ferromagnetic layer, a non-magnetic layer, and another ferromagnetic layer. The GMR element utilizes the magneto-resistive effect of what is called a spin-valve film. The magnetization of one of the ferromagnetic layers is fixed by applying an exchange bias to the same, while the magnetization of the other ferromagnetic layer is varied by external magnetic fields. Any change in the relative angle between the directions of magnetization of the two ferromagnetic layers can be detected as a resistance change. Such GMR elements developed so far include CIP (Current-In-Plane) GMR elements in which a current is passed in the plane of a spin-valve film to detect a resistance change, and CPP (Current-Perpendicular-to-Plane) GMR elements in which a current is passed perpendicularly to the plane of a spin-valve film to detect a resistance change. Both of CIP GMR elements and CPP GMR elements have a magneto-resistance ratio (MR ratio) on the order of a few percents, and they are considered to allow a recording density of about 200 Gbits/inch2.
TMR elements utilizing the tunneling magneto-resistive effect (TMR effect) are currently being developed to allow magnetic recording at still higher densities. A TMR element comprises a multi-layer film comprising a ferromagnetic layer, an insulation layer, and another ferromagnetic layer. A tunneling current is passed by applying a voltage between the ferromagnetic layers. The TMR element takes advantage of the fact that the magnitude of a tunneling current changes depending on the directions of magnetization of top and bottom ferromagnetic layers, so that a change in the relative angle between the magnetizations is detected as a change in tunneling resistance. TMR elements having a maximum MR ratio about 50% have become available. TMR elements provide signal magnitude (voltages or current) higher than those of GMR elements because they have higher MR ratios.
However, a higher signal magnitude results in an increase not only in pure signal components, but also in noise components attributable to shot noises. Therefore, the signal-to-noise ratio (SN ratio) becomes difficult to achieve. Shot noises are attributable to current fluctuations generated when electrons irregularly pass a tunnel barrier, and they increase in proportion to the square root of tunneling resistance. Therefore, the tunnel insulation layer must be made thin to reduce tunneling resistance in order to suppress shot noise so that the TMR element can obtain a required signal (voltage or current).
The size of TMR element must be reduced to be closer to the recording bit size in order to obtain higher recording density. It is therefore required to make the tunnel insulation layer thinner in order to reduct the junction resistance. For a recording density of 300 Gbits/inch2, junction resistance must be 1 Ω·cm2 or less, and a tunnel insulation layer having a thickness equivalent to two atomic layers must be formed in the case of an Al—O (aluminum oxide film) tunnel insulation layer. Such thin layer is drastically difficult to fabricate. Also On the other hand, the possibility of shorting between top and bottom electrodes and thus the danger of reduction in SN ratio becomes higher too. For the above-described reasons, the limit of the recording density by TMR elements is estimated at 300 Gbits/inch2.
Any of the above-described elements utilizes magneto-resistive effects in a broad sense, and magnetic white noises commonly encountered in those elements is significant. Those noises become more dominant as elements become finer because they are attributable to thermal fluctuation of magnetization unlike electrical noises such as shot noise described above. It is assumed that white noises exceed electrical noises in elements which allow recording densities of 500 Gbpsi and higher.
The use of a micro magnetic oscillation element having sensitivity higher than that of GMR type elements according to Patent Document JP-A-2006-86508, is recently suggested for the purpose of avoiding magnetic white noises, and improving the recording density of magnetic recording further. However, a problem still remains in magnetic sensors utilizing such micro magnetic oscillation elements. A magnetic layer at a sensing part must be processed into dimensions on the order of the bit size of the recording medium (a thickness dimension corresponding to a bit width and an aperture width corresponding to a track width). Therefore, processing techniques for making an element in a fine pillar-like shape become more difficult to implement. Reduction in SN ratio attributable to magnetic thermal noises constitutes another problem.